Generally, “hybrid vehicle” is a vehicle utilizing a plurality of power sources, and usually refers to a hybrid electric vehicle (HEV) that is driven by an engine and a motor. HEVs may be realized in various schemes adopting an engine and a motor, and a majority of schemes are based on a parallel construction or a series construction.
A series scheme is simpler in structure than a parallel scheme, so it is easier to control. However, series HEVs are less energy efficient than parallel HEVs because energy in series HEVs is first converted from mechanical energy of an engine into electrical energy in a battery, and then used for running a motor. Parallel HEVs, though more complicated in structure and control, are more energy efficient because mechanical energy of an engine and electrical energy of a battery may be simultaneously used. For this reason, a parallel scheme is usually adopted for a passenger car.
A series HEV, though less energy efficient than a parallel HEV, can always operate the engine at an optimal operating point. However, a parallel HEV cannot always operate the engine at an optimal operating point since the engine and the motor are mechanically coupled together through a transmission and the engine speed is correlated with the vehicle speed. Consequently, operating efficiency of an engine varies according to vehicle speed. In order to solve this problem, a continuously variable transmission (CVT) utilizing a metal belt is usually considered a favorable transmission because it enables the engine speed to be controlled independently from the vehicle speed. However, such a CVT requires very high hydraulic pressure for operation in comparison with other transmissions such as an automatic transmission. Therefore, in spite of various functionally favorable features, a CVT does not manifest particularly substantial energy efficiency.
On the other hand, in a double clutch transmission (DCT), torque from an engine is transmitted to two input shafts through two clutches, and is then changed and output using gears associated with the two input shafts. By adapting two clutches and an automatically shifting device to a scheme similar to a conventional manual transmission, the convenience of an automatic transmission may thusly be obtained while maintaining the efficiency level of a manual transmission.
Therefore, if such a DCT may be adapted to an HEV as its transmission system, the HEV's efficiency may be further enhanced from an HEV that uses a CVT.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention, and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.